IONISATION ENERGY
Reference has already been made to Lothar Meyer's plot of "atomic
volume' against atomic weight as a demonstration of a physical
property of the elements and Figure L5 shows a modem plot of
'atomic volume' against atomic number. Although regularities are
clearly observable "atomic volume' has no single meaning for all the
elements—certainly it does not measure atomic size, a quantity which
depends on the state of aggregation of the element. There are, however,
more fundamental physical properties which show periodicity.